Prostheses, for example, hip prostheses, exhibit a shank of the prosthesis which is anchored in the bone by means of bone cement. To accomplish this a cavity is drilled in the bone and partially filled with bone cement and subsequently the shank of the prosthesis is introduced into the cavity in the bone. During the introduction of the shank, the bone cement is displaced. The cement is urged towards the opening and is forced out through the gap between the cavity in the bone and the shank of the prosthesis. The bone cement is usually obstructed against flowing out in order to keep the bone cement under a certain pressure so that the bone cement also penetrates into small pores and crevices. In order to arrive at a sealing action, the practice is known of laying a soft disc of plastics on the opening in the bone and to pierce it during the introduction of the shank of the prosthesis. This kind of sealing exhibits various disadvantages. The sealing action depends upon how the soft disc is pierced. The sealing action and hence the pressure in the bone cement is not reproducible. In addition, the introduction of the shank of the prosthesis into the cavity in the bone is obstructed, since directly before the introduction the seal has to be pierced.